1. Field of the Invention
This invention pertains to improvements in preformed crowns. More specifically, the invention is the application of a veneer over a stainless steel crown for children. The veneer is comprised of an esthetically pleasing material, which more closely matches color of the children's natural teeth, with the added benefit of being thin and yet having superior resistance to the stresses to which crowns are typically subjected.
2. State of the Art
Performed stainless steel crowns (SSCs) are still the preferred choice for whole or partial replacements of carious teeth in children. Dental composites provide an alternative, which resemble teeth, but lack the strength of a preformed SSC. Children with extensive caries, occlusions with deep anterior bites, or that bruxes (teeth grinding habits) need the strength of stainless steel crowns.
A typical SSC is constructed from a preformed base material crown 10 such as stainless steel which is placed in the mouth to cover a prepared tooth 12 as shown in FIG. 1. The prepared tooth 12 is shown as having its surface ground away sufficient for the placement of the crown 10 thereon. The scale of the teeth shown and the crown 10 to be placed thereon is for ease of illustration and should not be considered to be at the correct scale. Furthermore, the portion of the tooth 12, which has been ground away, is also for illustration purposes only.
As shown in FIG. 1, the base metal crown 10, which as shown for illustration purposes is not a molar, and therefore can be pictured generally as a flattened bowl which is formed in the shape of a tooth with an open end 16 for placement over the prepared tooth 12. Proper tooth preparation includes removing all caries and proper shaping of the remaining natural tooth 12 to receive the SSC 10. Therefore, the prepared tooth 12 is typically left in place in the mouth so that its root provides anchor in the jaw for the SSC. An esthetic SSC, however, also includes a veneer placed over the base metal 10. It is the veneer and the method of attachment of the veneer to the base metal, which is critical to the success of the esthetic SSC, and the subject matter of this patent.
The esthetic SSC is designed with several objectives in mind. One purpose is to hide the base material covering the natural tooth so that a more natural looking artificial tooth is seen. The esthetic SSC also makes contact with other teeth or dental work, as well as food items placed in the mouth. Consequently, the preformed crown with the veneer in place must be able to withstand great shearing stresses because of the contact with forces of occlusion.
Similarly, a bridge is an artificial tooth replacement, which spans a gap between natural teeth. Unlike the esthetic SSC, however, the bridge can be fixed or removed. The bridge is typically supported by natural teeth or roots adjacent to the space being filled by the bridge.
It was stated that the esthetic SSC or bridge structure (hereinafter referred to only as an esthetic SSC) is comprised of a veneer bonded to a base metal beneath. However, the veneer can have several layers, which exhibit different light absorption an reflective properties. The different properties are obtained by mixing the veneers with other materials. For example, many of the esthetic SSCs used today have a decidedly unnatural look to them. Some have the appearance of a flat paint, lacking any translucency of natural enamel. This makes the crown stand out and be very noticeable and visible.
The state of the art is replete with various tooth colored materials used as aesthetic veneers to ameliorate the silver crown smile that many people have difficulty accepting, especially children. Children are especially sensitive to their appearance, and their self-esteem can be profoundly affected by the comments of their peers who are quick to point out distinguishing features.
The majority of the materials used as veneers use dental composites that are bonded onto esthetic SSCs. The bonding process is also a major obstacle to reliable esthetic SSC construction because it is another place where failure of the esthetic SSC can occur. State of the art bonding can consist mechanically of sandblasting, welding of mesh, and making holes in the crown, as well as chemical retention using silanes, tin plating, or the use of bonding agents such as 4-meta. Another example is taught in U.S. Pat. No. 4,722,689 issued to Corbett, whereby epoxy is electrically deposited onto temporary crowns.
Another disadvantage of the prior art is that once a veneer or facing is fractured, the veneer typically continues to separate further, or the dental construct might break apart completely due to percolation at the interface between the veneer and the base metal.
All of the veneers and bonding methods described above suffer at least one serious inherent weaknesses such as bonding strength, staining, wear under abrasion, general unattractiveness or lack of esthetic consideration. It would therefore be an advantage over the prior art to create an esthetic SSC to be used in a child's mouth, which has the appearance of natural teeth while providing superior abrasion resistance. It would also be an advantage to bond a veneer to the base metal using a mesh or mechanical retention having the bond strength contingent upon welded stainless steel rather than upon the flexural strength or elastic limit of the material of the veneer. That is to veneer a material so strong, that the point of failure does not remain with the porcelain or plastic resin but with the failure of stainless steel mesh that is welded to the base metal. This is in fact a very superior bond of the veneer to the crown. Still another advantage, would be to have a veneer that does not degrade and chip away during its short life in the mouth as a restoration on a baby tooth.